The power saving obtained in two-stage compression with intercooling is a well known fact but double-staging is usually applied for high pressure ratios and for important size units. The ever increasing energy cost leads to the increased use of two-stage compressors in the field of modest pressure ratios such as encountered in shop air requirements and in that of small units. Another factor to influence selection of the compressors, pumps, etc., is the increased use of variable speed a.c. electrical motors which may provide power savings by varying the compressor speed in response to the requested output. These two factors are unlikely to favour the existing rotary compressors available on the market i.e. the oil flooded vane type and the dry or oil flooded screw type.
Vane compressors already available in two-stage designs suffer from several drawbacks: high leakage area, hence high oil circulation for a given displacement; high rubbing speeds; vane sticking problems which imply the necessity of operating within a narrow range of rotational speed; and an overall high power consumption.
Screw compressors, although popular, are seldom used for two-stage operations because they usually require two complete separate units sometimes interconnected via an intercooler with the result that this leads to an expensive arrangement. Moreover, its performance is dependent upon its rotational speed and, therefore, output control via speed control will not produce any significant power saving.
The above mentioned techniques would favour the classical reciprocating compressor which, when properly designed, has an inherent ability to operate over a wider range of speeds and has less power consumption than the equivalent two rotary volumetric compressors.
A rotary compressor with less leakage area than that in the vane compressor, with double-staging at reasonable cost and with the ability to operate at various speeds while maintaining a satisfactory performance, will maintain a continued competitive edge over the reciprocating compressor.